From the blog post announcing the game:
"Pre-Order Poker Night at the Inventory now and receive 25% off additional purchases in the Telltale store."
It lowered my total (before tax and shipping) from 69.80 to 53.59 (I also bought all the Devil's Playhouse prints and the Sam & Max symbiotes)... not bad!

I used it to buy Devil's Playhouse prints and a bunch of T-shirts.
Now I still have unused the 20% discount that came with S&M304. I hope I can use it with BttF if it comes out in October, near the release of the movies on Blu-Ray.
If not, I hope I'll find anything else to use the discount with before 31 December...

Now I still have unused the 20% discount that came with S&M304. I hope I can use it with BttF if it comes out in October, near the release of the movies on Blu-Ray.

Unfortunately, if I remember correctly, the discount coupons are only good for items that were released BEFORE the coupon was issued, which means they're not eligible for future releases. I'm not sure if what I read applies to all Telltale coupons or just specific ones, so you may want to check with Telltale for a definitive answer.

Unfortunately, if I remember correctly, the discount coupons are only good for items that were released BEFORE the coupon was issued, which means they're not eligible for future releases.

I'm pretty sure that's only when the coupon is for a free game.
Anyway, I tried the S&M 304 20% off coupon and it worked.
I did not use it, though as the 25% off for pre-ordering Poker Night at the Inventory was an even better deal.

EDIT: To clarify, I tried the 20% off coupon with The Devil's Playground prints and the Sam & Max symbiotes, items which were released after the coupon was issued.

The S&M 304 20% off coupon says it's a discount for the whole order, and I didn't find any disclaimer saying it's not valid for (back to the) future games.
But even if games don't enter on the discount, there's still stuff on the store that I haven't yet and that I may be interested on (Book of Grickle and other items...)
Anyway, now that I read the coupon more carefully, I'm thinking that maybe I'm not eligible for that discount anymore. It says "we're giving you 20% off your next order", and I've already placed an order with Poker Night and other stuff, so anything I could try to buy using that coupon tecnically won't be my next order...

I'm pretty sure that's only when the coupon is for a free game.
Anyway, I tried the S&M 304 20% off coupon and it worked.
I did not use it, though as the 25% off for pre-ordering Poker Night at the Inventory was an even better deal.

EDIT: To clarify, I tried the 20% off coupon with The Devil's Playground prints and the Sam & Max symbiotes, items which were released after the coupon was issued.